(1) Field of the Invention
The present invention relates to a nickel electrode for an alkaline storage battery and a method of manufacturing the nickel electrode. In particular, the present invention relates to improvements in active material for use in the nickel electrode.
(2) Prior Art
Alkaline storage batteries using conventional nickel electrodes have the following problem. As the charging reaction and the gaseous oxygen generating reaction that occur in such an alkaline storage battery requires similar electric potentials, the oxygen over potential (which means the potential difference between the charging reaction and the gaseous oxygen generating reaction in this specification) decreases at high temperatures. This lowers the charging efficiency of the alkaline storage battery.
In view of such a problem, the following techniques to increase the oxygen over potential have been devised.
One of them is a technique to mix cobalt with nickel hydroxide to form a solid solution and use the solid solution as active material. However, if a large amount of cobalt is used, the operating voltage of the alkaline storage battery decreases. Furthermore, the manufacturing cost of such an alkaline storage battery is high, as cobalt is expensive.
Japanese Laid-Open Patent Application No. H11-73957 discloses a technique for uniformly mixing nickel, cobalt, and yttrium in a nickel electrode for an alkali storage battery. However, the charging efficiency of this alkaline storage battery at high temperatures does not improve significantly.
Also, Japanese Laid-Open Patent Application No. H10-125318 discloses a technique for covering a surface of the active material mainly composed of nickel hydroxide with a layer of an independent crystal. The independent crystal is a solid solution mainly composed of nickel hydroxide active material particles, and contains elements selected from the group consisting of Mg, Ca, Al, Sr, and the like, and elements selected from the group consisting of Co and Mn. However, generally, the active material for a nickel electrode should include as few elements other than nickel as possible, in view of not lowering the energy density of the alkaline storage battery. Moreover, only the elements selected from the group consisting of Mg, Ca, Al, Sr, and the like can have the effect of preventing the charging efficiency of the alkaline storage battery from being lowered at high temperatures, therefore, the elements selected from the group consisting of Co and Mn do not need to be used. Furthermore, the presence of cobalt on the surface of the active material lowers the operating voltage of the alkaline storage battery. Therefore, it is not preferable to use cobalt in the layer covering the active material.
Also, Japanese Laid-Open Patent Application No. H10-149821 discloses a technique for forming active material having a dual-layer structure, in such a manner that the upper layer in the active material has a different composition from the lower layer of the active material. The upper layer contains high density of Ca, Ti, and the like, whereas the lower layer contains high density of Al, V, and the like. However, as the main component of the upper layer is nickel, the elements such as Ca and Ti have little effect of improving the charging efficiency of an alkaline storage battery at high temperatures.
In view of the above problems, the present invention aims to provide a nickel electrode for use in an alkaline storage battery that exhibits excellent charging characteristics at high temperatures with relatively low manufacturing cost. The present invention also aims to provide a method of manufacturing the nickel electrode.
As explained above, the technique for including cobalt and elements selected from the group consisting of Ca, Al, Sr, Sc, Y, and Ln (lanthanoide series) in the nickel electrode is well-known. However, the inventors of the present application have made thorough studies to obtain a nickel electrode that would provide an alkaline storage battery with high operating voltage and prevent the charging efficiency from being lowered at high temperatures when employed in the alkaline storage battery. As a result, the inventors have come up with the necessity of designating the locations and the compounding ratios of the above listed elements in the nickel electrode in the following way.
As for cobalt, instead of including it on the surface of the active material mainly composed of nickel, by mixing it into the active material in a state of a solid solution, the energy density of an alkaline storage battery is prevented from decreasing, and also the charging efficiency of the alkali storage battery is improved. On the other hand, problems remain as that the operating voltage of the alkaline storage battery is lowered in such a case, and that the manufacturing cost may increase if a large amount of cobalt is used. It is difficult to find an appropriate balance between the improved charging characteristics at high temperatures, the operating voltage, and the manufacturing cost.
As for the elements selected from the group consisting of Ca, Al, Sr, Sc, Y, and Ln, they are not necessarily mixed with nickel to form a solid solution for being used as active material, but may be made present at the interface between the electrolyte and the active material. In such a case, a smaller total amount of such elements can have the effect of increasing the oxygen overpotential. In short, the above listed elements should be retained on the surface of the active material.
Although the oxygen overpotential further increases if a larger total amount of the above listed elements is retained on the surface of the active material, these elements are not directly related to the charging and discharging reaction in the alkaline storage battery. They may rather have a negative effect of being a resistance force against the charging and discharging reaction. Also, in view of the energy density of the alkaline storage battery, it is not preferable to retain the large amount of such elements on the surface of the active material.
Accordingly, the amount of such elements to be retained on the surface of the active material, on the condition that the oxygen over potential increases while the energy density does not decrease, is as follows. The ratio of the elements selected from the group consisting of Ca, Al, Sr, Sc, Y, and Ln to be retained on the surface of the active material, expressed in molar percent of metal, should be in the range of 0.3% to 5% relative to the total amount of nickel contained in the active material.
Here, two methods can be considered for retaining the elements selected from the group consisting of Ca, Al, Sr, Sc, Y, and Ln, on the surface of the active material. First method is to retain only a compound of the elements on the surface of the active material (the active material being mainly composed of nickel hydroxide), without mixing the compound with other compounds. Second method is to form an eutectoid layer of a solid solution made by mixing (a) the compound of the above elements and (b) other compounds, on the surface of the active material (the active material being mainly composed of nickel hydroxide).
In the case of the second method, the elements selected from the group consisting of Ca, Al, Sr, Sc, Y, and Ln are less likely to come in contact with the electrolyte than in the case of the first method. In order for a larger amount of such elements to be present at the interface between the active material and the electrolyte, besides the ratio of the elements expressed in molar percent of metal being in the range of 0.3% to 5% relative to the total amount of nickel contained in the active material, it is preferable that the ratio of the elements to be retained on the surface of the active material, expressed in molar percent of metal, is at least 20% relative to the total amount of metals present therein.
As described above, the decreases in the charging efficiency of the alkaline storage battery at high temperatures can be prevented even when a small amount of cobalt is contained in the active material, by covering a surface of the active material with a layer made of the elements selected from the group consisting of Ca, Al, Sr, Sc, Y, and Ln. Therefore, by setting the ratio of cobalt to be contained in the part of the active material that is mainly composed of nickel compound and plays a main role in the charging and discharging reaction of the alkaline storage battery, expressed in molar percent of metal, in the range of 0.5% to 3% relative to the total amount of nickel contained in the active material, a nickel electrode that would provide an alkaline storage battery with high operating voltage and prevent the charging efficiency from being lowered when employed in the alkaline storage battery can be obtained at low manufacturing cost.
To achieve the stated object, the present invention aims to provide a nickel electrode for an alkaline storage battery including a conductive porous member; and an active material with which the conductive porous member is filled, the active material including (a) a main active material layer substantially made of nickel hydroxide, the main active material layer containing cobalt in a state of a solid solution, and (b) a compound layer that contains at least one element selected from the group consisting of calcium, aluminum, strontium, scandium, yttrium, and lanthanoide series, the compound layer being formed on a surface of the main active material layer, wherein a metal molar ratio of cobalt contained in the main active material layer to nickel contained in the main active material layer is in a range of 0.5% to 3.0% inclusive, and a metal molar ratio of the at least one element contained in the compound layer to nickel contained in the active material is in a range of 0.3% to 5.0% inclusive.
The present invention also provides a method of manufacturing a nickel electrode for an alkaline storage battery, including: a step for filling a conductive porous substrate with a main active material substantially made of nickel hydroxide, the main active material containing cobalt at a metal molar ratio of 0.5% to 3.0% in a state of a solid solution; and a step for forming a compound layer containing at least one element selected from the group consisting of calcium, aluminum, strontium, scandium, yttrium, and lanthanoide series, on a surface of the main active material.
The present invention also provides a method of manufacturing a nickel electrode for an alkaline storage battery, including: a step for forming an active material by forming a compound layer on a surface of a main active material, the main active material being substantially made of nickel hydroxide and containing cobalt at a metal molar ratio of 0.5% to 3.0% in a state of a solid solution, the compound layer containing at least one element selected from the group consisting of calcium, aluminum, strontium, scandium, yttrium, and lanthanoide series; and a step for filling a conductive porous substrate with the active material.